1. Field of the Invention
The present invention relates to a ratchet-type tensioner for applying an appropriate tension to a timing chain of a vehicle engine. More particularly, the present invention relates to a ratchet-type tensioner which employs an aluminum tensioner housing formed of aluminum or an aluminum alloy.
2. Description of the Related Art
Conventionally, a timing chain is used in a chain drive for transmitting rotational motion from a crankshaft to a camshaft of an automobile engine, and a ratchet-type tensioner is used for applying tension to the timing chain. That is, the tensioner is adapted to apply an appropriate tension to the slack side of the timing chain to thereby suppress vibration, which would otherwise occur in the timing chain when the chain travels, and to maintain a proper pressing force regardless of elongation of the chain.
The conventional ratchet-type tensioner is mounted on an engine body on the slack side of a chain (timing chain). The chain is fitted around and extends between a driving sprocket rotated by a crankshaft of an engine and a driven sprocket fixed on a camshaft.
The ratchet-type tensioner includes a plunger projecting from the front face of a tensioner housing such that the plunger can undergo reciprocating motion. A tip portion of the plunger presses the rear side of a tension lever at a position near the free end thereof. The tension lever is pivotally supported on the engine body by means of a support shaft. Through such an arrangement, a shoe surface of the tension lever is brought into slidable contact with the slack side of the chain to thereby apply tension to the chain.
The tensioner housing has a plunger accommodation bore into which the plunger is fitted such that the plunger may undergo reciprocating motion. The plunger has a hollow portion opening to an end surface of the plunger which faces the bottom of the plunger accommodation bore. A plunger spring is inserted into the hollow portion and disposed between the bottom of the plunger accommodation bore and that of the hollow portion, thereby urging the plunger at all times in such a direction that the plunger is projected from the tensioner housing.
A ratchet is pivotally supported on the tensioner housing by means of a ratchet pin. The ratchet has ratchet pawls which mesh with rack teeth formed on an outer surface of the plunger.
A ratchet spring is disposed between the tensioner housing and the ratchet and urges the ratchet to rotate about the ratchet pin so that the ratchet pawls are in engagement with the rack teeth at all times. Through engagement between the ratchet pawls and the rack teeth, the plunger is prevented from moving in a receding direction.
In the ratchet-type tensioner having the above-described structure, since the tensioner housing is made of gray cast iron, the ratchet-type tensioner is heavy.
In order to solve this drawback, the present invention employs a tensioner housing formed of an aluminum-containing metallic material such as aluminum or an aluminum alloy. However, mere employment of an aluminum-containing metallic material for the tensioner housing causes the following problem. A stress (load) corresponding to the tension of the chain acts on the plunger. Further, a side force (a force perpendicular to the advancing direction of the plunger) acts on the plunger so that the plunger is pressed against the sliding surface of the plunger accommodation bore. Therefore, when the tensioner housing is formed of an aluminum-containing metallic material, the friction resistance at the sliding surface increases, so that when the plunger reciprocates over a distance of about 0.1 to a few millimeters during rotation of the engine, seizure occurs between the tensioner housing and the plunger, and the strength of the plunger accommodation bore of the tensioner housing is reduced.